World's smallest steam engine comes to life

Kate Taylor, 12th December 2011

German physicists say they've built a heat engine measuring only a few micrometers across which works as well as a normal-sized version - although it sputters, they admit.

Researchers at the University of Stuttgart and the Max Planck Institute for Intelligent Systems say that the engine does basically work, meaning there's nothing, in principle, to prevent the construction of highly efficient, small heat engines.

"We've developed the world's smallest steam engine, or to be more precise the smallest Stirling engine, and found that the machine really does perform work," says Clemens Bechinger of the University of Stuttgart.

"This was not necessarily to be expected, because the machine is so small that its motion is hindered by microscopic processes which are of no consequence in the macroworld." The disturbances cause the micromachine to run rough and sputter.

The researchers couldn't construct the tiny engine in the same way as a normal-sized one. In the heat engine invented almost 200 years ago by Robert Stirling, a gas-filled cylinder is periodically heated and cooled so that the gas expands and contracts. This makes a piston execute a motion with which it can drive a wheel, for example.

However, the working gas in the new engine consists of just one individual plastic bead measuring three micrometers, which floats in water. Since the colloid particle is around 10,000 times larger than an atom, researchers can observe its motion directly in a microscope.

The physicists replaced the engine's piston with a focused laser beam whose intensity is periodically varied. The optical forces of the laser limit the motion of the plastic particle to a greater and a lesser degree, like the compression and expansion of the gas in the cylinder of a large heat engine. The particle then does work on the optical laser field.

In order for the contributions to the work not to cancel each other out during compression and expansion, these must take place at different temperatures. This is done by heating the system from the outside during the expansion process, just like the boiler of a steam engine. The researchers replaced the coal fire of an old-fashioned steam engine with a further laser beam that heats the water suddenly, but also lets it cool down as soon as it is switched off.

The fact that the Stuttgart machine runs rough is down to the water molecules which surround the plastic bead. The water molecules are in constant motion due to their temperature and continually collide with the microparticle. In these random collisions, the plastic particle constantly exchanges energy with its surroundings on the same order of magnitude as the micromachine converts energy into work.

"This effect means that the amount of energy gained varies greatly from cycle to cycle, and even brings the machine to a standstill in the extreme case," explains Stuttgart's Valentin Blickle. Since macroscopic machines convert around 20 orders of magnitude more energy, the tiny collision energies of the smallest particles in them aren't important, he says.

"Our experiments provide us with an initial insight into the energy balance of a heat engine operating in microscopic dimensions," says Bechinger. "Although our machine does not provide any useful work as yet, there are no thermodynamic obstacles, in principle, which prohibit this in small dimensions."